Galactic B-type supergiants

Abstract

The chemical composition of the atmospheres of the B-type supergiants is an important diagnostic of their evolutionary history, in particular with regard to the question of whether or not their atmospheres become contaminated by the products of the CNO-cycle due to mixing during a previous red-supergiant evolutionary phase. There is some circumstantial evidence for this scenario; i)a number of analyses of OB-supergiants have implied a helium enrichment of their atmospheres and ii)there is evidence that the precursor of SN1987A (a B3 I supergiant) in the LMC was surrounded by a nitrogen rich cloud thought to have been ejected while the star was a red-supergiant. With regard to i), there is little convincing quantitative evidence for the expected CNO abundance anomalies which should accompany a helium enrichment. There are a number of difficulties in this area, for example, curve-of-growth and LTE abundance analyses of B-supergiants have been obliged to adopt rather large microturbulent velocities (values range between 10km/s and 25km/s). There are also inconsistencies in the reproduction of the He I line profiles and these problems are not yet adequately resolved by the use of plane-parallel non-LTE model atmospheres and line formation calculations. A current survey of 46 bright galactic B-supergiants does imply a correlation between the strength of the N II lines and the weakness of the C II lines but there is also evidence that this trend is also correlated with increasing luminosity. It is also clear that the He I lines are also much stronger than non-LTE model predictions for all B-supergiants in the sample. It is possible that at least part of this effect is due to inadequacies in the current models for these stars, a possibility that is currently under investigation. A more complete description of this work may be found in;